Vast quantities of hydrogen will be required when this chemical replaces hydrocarbon liquids and gases as the world's primary fuel. The logical source for this hydrogen is H.sub.2 O, water, which covers over 70% of the earth's surface. When hydrogen is burned the original hydrogen source, water, is 100% reproduced. Thus, no depletion of the fuel resource occurs.
However, more energy is consumed than is produced in the form of hydrogen when water is decomposed. A prime energy source such as solar, fission or fusion energy is required. In order to minimize the energy demand on these intrinsically complex and expensive prime energy sources the thermal efficiency of the coupled water-splitting process must be maximized. This recognition has served as the impetus for a world-wide research and development effort to determine the most efficient means for splitting water into hydrogen and oxygen.
It has long been recognized that a potentially most efficient method for splitting water is that of using closed thermochemical cycles. In such closed cycles water is introduced as a reactant to one or more of several intermediate chemical reactions composing such a cycle, the sum of the reactions of which is H.sub.2 O.fwdarw.H.sub.2 +1/2O.sub.2. Such cycles can be more efficient in the consumption of process heat than is water electrolysis.
By the laws of thermodynamics it can be shown that for maximum efficiency a thermochemical cycle accepting heat at a specified high temperature and rejecting heat at a specified low temperature must be a two-reaction cycle. This principle, based on sound thermodynamics, stands inviolate and it is only because of the lack of success, to date, in the search for and development of a two-reaction cycle to full potential that much recent effort has focused on cycles involving three or more reactions, no matter the sacrifice in potential thermal efficiency that this involves.
U.S. Pat. No. 3,963,830 discloses a two-reaction process for splitting water into hydrogen and oxygen by the use of a hydrated zeolite. The hydrated zeolite is contained in a reaction vessel at high temperature where it is contacted by the water with a resultant endothermic redox reaction wherein dehydration of the zeolite occurs and oxygen gas is produced. Then the reaction vessel is cooled with a resultant exothermic redox reaction in which there is rehydration of the zeolite and wherein hydrogen gas is produced. The cycle is then repeated by reheating the reaction vessel and flowing in additional water to produce further oxygen gas while again dehydrating the zeolite, followed by again cooling the reaction vessel to produce further hydrogen gas and to again rehydrate the zeolite.
While this process disclosed in the patent does provide a means for nonelectrolytically splitting water in a two-reaction cycle, it has a number of deficiencies resulting in thermal inefficiencies of such a magnitude as to render the process noncompetitive with electrolysis or with thermochemical cycles involving three or more reactions. One of the most serious disadvantages is that the entire process is predicated on the dehydration and rehydration of the zeolite. Thermodynamic calculations, verified by laboratory measurements, establish that if the oxygen-producing reaction is predicated on dehydration, there is inherent thermal inefficiency in that the heat energy required to break the bonds of water of hydration is of sufficient magnitude to reduce the thermal efficiency of the cycle below 10%. For comparison, the thermal efficiency of modern water electrolysis is about 20%. Hence, if a zeolite based process for splitting water is to be competitive even with electrolytic processes, dehydration and rehydration of the zeolite must be eliminated.
Another serious deficiency of the process disclosed in the aforesaid patent, resulting in excessive thermal inefficiency, is that it requires energy input for the repeated cyclical heating and cooling of the reaction vessel and contents in which both the oxygen-producing and the hydrogen-producing reactions occur. The temperature differential required must exceed about 200.degree. C., and rejection of heat from the cycle during cool-down constitutes a heat loss of sufficient magnitude alone to reduce the thermal efficiency of the cycle so greatly as to be noncompetitive.
A still further deficiency of the process taught in the aforesaid patent is that it does not itself inherently produce the hydrogen and oxygen at the high pressure required for efficient storage and pipeline distribution. If, subsequent to generation of the oxygen and hydrogen, there is requirement to pressurize these gases in order to attain the pressures needed for efficient storage and distribution, then the energy input required for such subsequent pressurization can itself diminish the over-all efficiency to the point of being noncompetitive.